Field of the Invention
The present invention relates to antenna structures and more particularly to an antenna structure adapted to receive and transmit a radio frequency signal with circular polarization.
Description of the Prior Art
In general, wireless radio frequency signals are capable of exhibiting characteristics of linear polarization or circular polarization and therefore are designed to do so as needed. For example, according to the specifications of global positioning systems (GPS), all electromagnetic waves for use in defining a GPS-oriented positioning signal usually exhibit circular polarization characteristics. To receive a positioning signal with circular polarization characteristics, an antenna disposed at a receiving end for the positioning signal is a patch antenna or ceramic antenna for receiving a radio frequency signal with circular polarization characteristics to thereby ensure that the positioning signal can be well received during its transceiving process.
The aforesaid two antennas exhibit satisfactory circular polarization characteristics and therefore are applicable to GPS. However, the aforesaid two antennas will have an insatiable demand for bandwidth if the positioning signal received by them is also for use in a global navigation satellite system (GNSS). Furthermore, ceramic antennas are time-consuming and intricate to manufacture and difficult to modify when designed; as a result, their receiving and transmitting frequencies cannot be readily fine-tuned. As electronic devices nowadays show a trend toward being lightweight, thin and compact, antenna structure designers have to give considerations to the volume of an antenna, space to be taken up by the antenna, circular polarization characteristics and applicable systems.